There is considerable disagreement over
the classification of the Yellow-legged Gulls found in the Atlantic.
Dwight
classified all the forms found in Macronesia (Azores, Canaries and
Madeira) and in Atlantic Morocco as atlantis. Stegmann
extended the range of atlantis to also include the coast of
Portugal. Bannerman
also included Portugal and Morocco within the range of atlantis.
Since this time a number of partial studies have been made which
placed the Portuguese and Morocco birds with michahellis
(Mediterranean Yellow-legged Gull). A recent study by Yésou
even placed the Canaries and Madeiran birds with michahellis
but this relied upon misquotes from the literature and ignored
aspects such as biometrics, vocalisations and wingtip patterns.

From personal studies at many sites on
the Atlantic seaboard, the position of Dwight is verified to some
extent with the gulls of the Canaries, Madeira and the Atlantic coast
of Morocco forming a coherent group termed Southern Atlantic
Yellow-legged Gull -- Southern atlantis. To the north is found
a closely related form Iberian Atlantic Yellow-legged Gull -- Iberian
atlantis. The biometrics and vocalisations of these forms are
broadly similar and very different from those of michahellis, a
term best reserved for birds from the Mediterranean.

All of Atlantic Iberia is occupied by
Iberian atlantis except for the northern coast of Spain in
Cantabria and Pais Vasco. Here a form occurs called Cantabrican
Atlantic Yellow-legged Gull -- 'cantabricans' or bernisi
-- which is so similar to Herring Gulls in many respects that it
could even be classified as a form of argenteus.

Less is known about the Azores form but
much has been speculated upon its characteristics. It has a few
characteristics of its own and other characteristics are a mixture of
those found in Southern or Iberian atlantis. It is tentatively
considered as a form in its own right: Azorean atlantis. More
data is needed for an objective placement to be made.

In the Straits of Gibraltar (Andalucia
in Spain, Tanger in Morocco) intermediate forms occur between
atlantis and michahellis. This is the intersection zone
between the two forms.

Other current sources of information,
besides these web pages, include:

Robert Lewis' Lewis
Birds.. Bob also agreed with an extended
atlantis range after seeing Yellow-legged Gulls in Portugal
in the field.

Jonsson (1998). He also thought that
birds breeding on the Atlantic coast of the Iberian peninsula and
Morocco stand morphologically closer to atlantis and are
better treated as such.